Constants

Table of Contents

A constant is an identifier (name) for a simple value. As the name
suggests, that value cannot change during the execution of the
script (except for
magic constants, which aren't actually constants).
A constant is case-sensitive by default. By convention, constant
identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A
valid constant name starts with a letter or underscore, followed
by any number of letters, numbers, or underscores. As a regular
expression, it would be expressed thusly:
[a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]*

// This is valid, but should be avoided:// PHP may one day provide a magical constant// that will break your scriptdefine("__FOO__", "something");

?>

Note:
For our purposes here, a letter is a-z, A-Z, and the ASCII
characters from 127 through 255 (0x7f-0xff).

Like superglobals, the scope of a constant is global. You
can access constants anywhere in your script without regard to scope.
For more information on scope, read the manual section on
variable scope.